While a number of superconducting compounds have been discovered in the Tl--Ca--Ba--Cu--O system, analogous superconducting compounds with Ba replaced by Sr cannot generally be synthesized as pure phases. Both single and double Tl--O layer structure materials have been synthesized in the Tl--Ca--Ba--Cu--O system but only single Tl--O layer structure materials have been synthesized in the Tl--Ca--Sr--Cu--O system and the structures of these phases have been stabilized by the introduction of substituents.
Z. Y. Chen et al., Solid State Commun. 83, 895 (1992) disclose that samples with nominal composition TlSr.sub.2 CaCu.sub.2 O.sub.7-x A.sub.x wherein A is F and x=0.0-1.5 are nearly single-phase and were superconducting with transition temperatures of 35-50 K. For A is Cl and x=0-0.5 it was found that a small amount of Cl destroyed the superconductivity of the samples and prevented the formation of the 1212 phase.
The single Tl--O layer structure materials containing no calcium have the formula TlSr.sub.2 CuO.sub.5 and TlBa.sub.2 CuO.sub.5. A. K. Ganguli and M. A. Subramanian, J. Solid State Chem. 93, 250 (1991) disclose that TlSr.sub.2 CuO.sub.5 is metallic and no superconductivity was observed down to 4.2 K. Thus Applicants are not merely adding fluorine to a known superconductor. It has been found that the addition of another element, such as La or Nd is required to render the Tl--Sr--Cu--O compositions superconducting. M. A. Subramanian, Mat. Res. Bull. 25, 191 (1990) discloses that the oxides with the formula TlSr.sub.2-x R.sub.x CuO.sub.5 wherein R is La or Nd and x=0.6 to 1.0 are single-phase with tetragonal structure. These oxides are superconducting with transition temperatures in the range of 35-48 K. As for the barium-containing compositions H. C. Ku et al., Jpn. J. Appl. Phys. 28, L923 (1989) disclose that TlBa.sub.2 CuO.sub.5 is metallic with a metastable superconducting transition occurring with an onset around 25 K and zero restivity at 10 K. Again, the presence of La or Nd is required in order to achieve a stable superconducting composition. The oxides with the formula TlBa.sub.2-x R.sub.x CuO.sub.5 wherein R is La or Nd and x=0.2 to 0.6 have tetragonal structure and are superconducting with transition temperatures above 40 K.
The present invention provides novel superconductors of the Tl--M--Cu--O type without the presence of Ca, La, or Nd.